Python 101

A concise tutorial to a concise language

YDP

21st August 2016

Contents

  1. Background
  2. Python Variables, Types & Lists
  3. Basic Operators
  4. String Formatting & Operations
  5. Conditions & loops
  6. Functions
  7. Classes and Objects
  8. Files - I/O operations
  9. Dictionaries, tuples
  10. Modules & Packages

A quick background

  • Multipurpose
    • Web, GUI, Scripting
  • Object oriented
  • Interpreted
  • Focus on readability & productivity

Is Python used enough?

Who uses Python?

  • Google
  • NASA
  • ebay
  • Quora
  • Reddit
  • YouTube
  • Mozilla Firefox

Hello World!


print "Hello  World!"

hello.py

Variables & Types

  • Numbers
    • Integers
    • Float
  • Strings
  • Null
one = 1
two = 2
three = one + two

hello = "hello"
world = "world"
helloworld = hello + world

Mixing operators between numbers and strings is not supported

Lists

  • Similar to arrays, but heterogeneous
  • Appending to a list
  • Extending a list
  • Accessing list items
mylist = []
mylist.append(1)
mylist.append("hello") 

print(mylist[1])

colors = ['red', 'brown', 'blue', 'green', 'white']

colors[1]
#brown

colors[2:4]
#['blue', 'green']

Basic Operators

  • Addition, multiplication,
  • Subtraction, division
  • Modulus (%)
  • Power operator
  • Strings with operators
  • Lists with operators
number = 1 + 2 * 10 / 5.0

remainder = 11 % 5

squared = 5 ** 2
cubed = 5 ** 3

#string operations
helloworld = "hello" + "world"
lotsofhellos = "hello" * 10 

#list operations
even_numbers = [2,4,6,8]
odd_numbers = [1,3,5,7]
all_numbers = odd_numbers + even_numbers

print [1,2,3] * 3

Exercise

The Meal

You've finished eating at a restaurant, and received this bill:

  • Cost of meal: $44.50

  • Restaurant tax: 6.75%

  • Tip: 15%

You'll apply the tip to the overall cost of the meal (including tax). Display the tip calculated.

Basic Operators

  • Addition, multiplication,
  • Subtraction, division
  • Modulus (%)
  • Power operator
  • Strings with operators
  • Lists with operators
number = 1 + 2 * 10 / 5.0

remainder = 11 % 5

squared = 5 ** 2
cubed = 5 ** 3

#string operations
helloworld = "hello" + "world"
lotsofhellos = "hello" * 10 

#list operations
even_numbers = [2,4,6,8]
odd_numbers = [1,3,5,7]
all_numbers = odd_numbers + even_numbers

print [1,2,3] * 3

String formatting

  • C style formatting
  • The % operator
  • () parenthesis
  • Argument specifiers:
    • %s - Strings
    • %f - floating point
    • %d - integers
    • %.<digits>f - Floating point number with fixed digits
name = "John"
age = 23
print "%s is %d years old." % (name, age)
# John is 23 years old


# This prints out: A list: [1, 2, 3]
mylist = [1,2,3]
print "A list: %s" % mylist

String Operations

  • Wide variety of operations like:
    • len
    • index
    • count
    • slicing
    • reverse
    • [start:stop:step]
    • upper/lower
    • split
s = "Hey there! what should this string be?"
print "Length of s = %d" % len(s)

print "The first occurrence of the letter a = %d" % s.index("a")

print "a occurs %d times" % s.count("a")

# Slicing the string into bits
print "The first five characters are '%s'" % s[:5] # Start to 5
print "The next five characters are '%s'" % s[5:10] # 5 to 10
print "The twelfth character is '%s'" % s[12] # Just number 12
print "The characters with odd index are '%s' " %s[1::2] #(0-based indexing)
print "The last five characters are '%s'" % s[-5:] # 5th-from-last to end

# Convert everything to uppercase
print "String in uppercase: %s" % s.upper()

# Convert everything to lowercase
print "String in lowercase: %s" % s.lower()

# Check how a string starts
if s.startswith("Str"):
    print "String starts with 'Str'. Good!"

# Check how a string ends
if s.endswith("ome!"):
    print "String ends with 'ome!'. Good!"

# Split the string into three separate strings,
# each containing only a word
print "Split the words of the string: %s" % s.split(" ")

Exercise

You will need to write a format string which prints out the data using the following syntax:

Hello John Doe! Your current balance is 53.44$.

  • Boolean values True and False
  • Conditions help us select from options
  • what if?
  • == and !=
  • The in operator
  • The is operator v/s == operator
  • The not operator
  • Indentation
name = "Ram"
if name in ["Ram", "Bharat"]:
    print "Your name is either Ram or Bharat."

people = 30
cars = 40

if cars > people:
    print "We should take the cars."
elif cars < people:
    print "We should not take the cars."
else:
    print "We can't decide."

print not False # Prints out True

Conditions

Loops

  • Loops help us iterate over a piece of code
    • for loop
    • while loop
  • Iterating over a list of items
  • Iterating over a sequence of numbers
primes = [2, 3, 5, 7]
for prime in primes:
    print prime #prime acts as iterator

# we can also build lists, first start with an empty one
elements = []

for i in range(0, 6):
    elements.append(i)

for x in range(3, 6):
    print x

Loops - while

  • Repeats as long as a certain Boolean condition is met
  • break
  • continue
  • The else part for loops
count = 0
while count < 5:
    print count
    count += 1  


# Prints out only odd numbers - 1,3,5,7,9
for x in xrange(10):
    # Check if x is even
    if x % 2 == 0:
        continue
    print x




Console Input

#Two types - input() and raw_input()

username = raw_input('What is your name? ')

#only for strings

myage = input("What is your age? ")

#all types except string




Exercise - Pig Latin

In this exercise, you have to write a Pig Latin translator.

Pig Latin is a language game, where you move the first letter of the word to the end and add "ay."  If a word begins with a vowel, just add “way” in the end. So "Python" becomes "ythonpay." “object” becomes “objectway”, and so on.

To write a Pig Latin translator in Python, here are the steps we'll need to take:

  1. Ask the user to input a word in English.

  2. Convert the word from English to Pig Latin.

  3. Display the translation result.

Functions

  • Functions are a convenient way to divide your code into useful blocks
  • Defining a function 
  • Passing arguments 
  • Returning from a function
  • Calling a function
def my_function():
    print "Hello there!"

def sum_two_numbers(a, b):
    return a + b

#Prints "Hello there!"
my_function()

x=sum_two_numbers(10,15)
#x now holds the sum of 10 and 15
print x

Exercise - Twin Primes

Twin prime numbers are a pair of prime numbers that have a difference of 2 between them. For example, (3,5), (5,7), (11,13), (29,31), etc.

Given a number n, display the first n pairs of twin primes
To do the above,

  1. Create a function prime(n) that checks if a number is prime or not

  2. Find a prime number and check if a twin prime exists for it

  3. Find n such pairs

Classes and Objects

  • Objects are an encapsulation of variables and functions
  • Class is a template for an object
  • Defining a class 
  • Creating objects
  • Accessing object variables and functions
class MyClass(object):
    variable = "blah"

    def function(self):
        print "This is a message inside the class."
		
myobjectx = MyClass()

myobjecty = MyClass()
myobjecty.variable = "yackity"
print myobjectx.variable   # This would print "blah".
print myobjecty.variable   # This would print "yackity".

myobjectx.function()

print type(myobjectx)

Exercise

In this exercise, we are going to create a banking system.

Create a class called Account that represents a customer account. It must contain the following variables and functions:

  • Account Type- savings, personal or corporate
  • User name- A string
  • Account number- A number that uniquely identifies each account. This must be auto-generated
  • Current balance- a floating point value that represents the current balance
  • displayBal()- A function that displays the current balance
  • DepositCash(amt) - A function that adds ‘amt’ to the current balance and updates it. In the case of savings accounts, the current balance cannot fall below 500
  • DrawCash(amt) - A function that carries out cash withdrawals of ‘amt’ from the current balance. The funds must first be checked and then updated

Create two objects of type Account:

  • ‘Acct1’ is a savings account that belongs to “Mishra” and has a current balance of Rs. 10000.00. He makes a deposit of Rs. 1500.00 into it, followed by a withdrawal of Rs. 7000.
  • ‘Acct2’ is a personal account that belongs to “Chopra” and has  a current balance of Rs. 5000. He makes 2 withdrawals of Rs. 4000 and Rs. 2000 each.

File I/O operations

  • Opening and closing a file
  • Reading from a file
  • Writing to a file
  • Appending to a file
f = open("test.txt", "r") 
#opens file with name of "test.txt" for read-only
print(f.read(10)) 
#Reads 10 characters and prints them or the entire line if number of characters is not specified
print(f.readLine())
#Reads an entire line and prints it

my_list = [i**2 for i in range(1,11)]
# Generates a list of squares of the numbers 1 - 10

f = open("output.txt", "w")
#opens file with name of "output.txt" in read-write
for item in my_list:
    f.write(str(item) + "\n")
f.close()

f = open("output.txt", "a")
#opens file with name of "output.txt" in append mode
f.write("Goodbye!")
#Appends "Goodbye!" to the end of the file
f.close()







Dictionaries 

  • Like arrays, but but works with keys and values instead of indexes
  • Creating dictionaries
  • Iterating
  • Removing values
phonebook = {
    "John" : 938477566,
    "Jack" : 938377264,
    "Jill" : 947662781
    "Jake" : 998833221
}

#Prints all the phone numbers in the dictionary
for name, number in phonebook.iteritems():
    print "Phone number of %s is %d" % (name, number)

#Deletes the entries of John and Jake from the phonebook
if "John" not in phonebook:
    print "John is not listed in the phonebook."
else:
    del phonebook["John"]

phonebook.pop("Jake")






Exercise

Given a list containing the key-words searched on Google by a user, count the number of occurrences of each key-word and display in the form of an array

For example, given this input -

Key_word_list = [‘cricket’, ‘india’, ‘football’, ‘cricket’, ‘yahoo’, ‘google’, ‘india’, ‘cricket’]

We should get this as the output:

cricket - 3, india - 2, football - 1, yahoo - 1, google -1

 

 

Tuples

  • Like lists, but cannot be changed
myList = [1,2,3]

myList.append(4)

print (myList)



myTuple = (1,2,3)

print (myTuple)

myTuple.append(4)
#Generates an error!

print (myTuple)

Modules

  • A module is a .py file containing Python definitions and statements
  • Importing modules
  • Writing your own modules
# Fibonacci numbers module

# write Fibonacci series up to n
def fib(n):    
    a, b = 0, 1
    while b < n:
        print b,
        a, b = b, a+b

# return Fibonacci series up to n
def fib2(n):   
    result = []
    a, b = 0, 1
    while b < n:
        result.append(b)
        a, b = b, a+b
    return result

>>> import fibo
>>> fibo.fib(1000)
1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987                                                                                                             
>>> fibo.fib2(100)                                                                                                                                                                                       
[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]
>>> fibo.__name__
'fibo'

Packages

  • Packages contain multiple packages and modules themselves
  • For ex, the module name A.B designates a submodule named B in a package named A
  • Like directories, but with a twist
#Package foo

#Defines packages & modules in foo
__init__.py:

#a list of packages and modules                                                                                                                                                                                                                                                                                                                                                        
__all__ = ["bar"]


>>> import foo.bar                                                                                                                                                                                                                                 
>>> from foo import bar









Thank You!

Questions?

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By Abhiram Ravikumar

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